Variable aperture for infra-red cameras

ABSTRACT

A variable aperture assembly for an infrared camera for use with optics having two or more f-numbers. The variable aperture assembly includes a variable iris disposed substantially along an axis of the infrared camera and is thermally connected to a cooling assembly. The variable iris is capable of causing an infrared radiation admitting opening to vary in size. The infrared radiation admitting openings of the variable aperture assembly can be circular or non-circular, including generally rectangular or racetrack shaped openings. The variable aperture assembly provides f-number matching by adjusting the infrared radiation admitting opening to the size necessary to effectively match the numerous f-numbers of the optics, or by adjusting to a size so that a fixed aperture on the radiation shield becomes the effective aperture stop, providing the f-number matching.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application that claims priority toU.S. application Ser. No. 12/192,069 filed on Aug. 14, 2008, now U.S.Pat. No. 7,816,650 entitled “External Variable Aperture and Relay forInfra-Red Cameras”, which prior application is a continuationapplication of U.S. application Ser. No. 11/273,919 filed on Nov. 14,2005, entitled “Cryogenically Cooled Adjustable Apertures for Infra-RedCameras” that issued as U.S. Pat. No. 7,427,758, which prior applicationis a continuation-in-part and claims priority to U.S. application Ser.No. 10/250,016 filed on May 28, 2003, entitled “Method and Apparatus forUsing Temperature Controlled Variable Diaphragms or Swappable FixedApertures with Infrared Cameras” that issued as U.S. Pat. No. 7,157,706,which application is incorporated by reference in its entirety and isclaimed priority thereto for this application. Applicant claims thebenefit of 35 U.S.C. section 120 with respect to the above mentionedforegoing applications.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with U.S. Government support under SBIR ContractNos. DAAB07-03-C-P004 and DAAB07-02-C-H304 awarded by the U.S.Department of the Defense. The U.S. Government has certain rights inthis invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to infra-red cameras and, more particularly, tothe optical or pre-photocell system and variable apertures for suchcameras.

2. Background

Thermal infrared radiation is the emission of photons by any and allobjects having a temperature that's above absolute zero. Those emissionsmay be captured by a thermal infrared camera (alternatively, knownsimply as an infrared camera), which is well known in the art. Aninfrared sensitive photocell is central to all infrared cameras. Aphotocell means any device sensitive to infrared radiation, including asingle element detector, a linear array and a two dimensional array ofdetectors (such as a 2D array of IR sensitive pixels) of variousinfrared sensitive materials. In infrared cameras the photocell is atwo-dimensional array of infrared sensitive pixels. In use, thephotocell, highly sensitive to thermal infrared radiation (hereinafterfrequently referred to simply as “radiation”) is exposed to radiationemanating from the object or scene being imaged. However, because thecamera enclosure is above absolute zero in temperature, the enclosureconcurrently emits radiation that is able to reach the photocell. Thatadditional radiation is not desired, since that radiation negativelyaffects the operation of the camera. To minimize or eliminate thateffect, the photocell is enclosed within a cold structure (referred toas a “radiation shield”).

The design of the radiation shield is dictated simply: if an observerwere to look out from the photocell, anything the observer could seewould emit radiation that would be incident on the camera photocell. Inorder to block the undesired radiation, the radiation shield must be theonly internal camera structure that the photocell is able to “see.” Theradiation shield shouldn't emit an excess of radiation. The “cold stop,”which is simply a name for a cooled aperture, provides the only paththrough the focusing optics for external radiation to reach thephotocell. The cold stop size is a compromise between the effectivenessof blocking the unwanted radiation (requiring a small aperture) andexcessive vignetting (requiring a large aperture). An ideal cold stopposition exists at the exit pupil of the lens. At that location the sizeof the cold stop is equal to the size of the exit pupil of the frontoptics, producing 100% effectiveness and no vignetting.

An active cooler, typically, such as a Peltier cooler, is oftenintegrated into the camera to keep the photocell and other components ofthe infrared camera cool. Typically, in order to control the unwantedradiation seen on the photocell, the cooling system must maintain a lowfixed temperature. Ideally, the temperature of the radiation shield iscold enough to produce only a negligible amount of radiation at thephotocell. That fixed temperature has a known effect on the photocelland that effect can be removed through image post processing. Thephotocell is also cooled to improve its radiation sensitivity and reducethe internally generated current, as the higher the temperature of thephotocell, the lower its usable dynamic range. However, in moresensitive systems the system must be cooled to as low a temperature asreasonably possible to minimize any unwanted radiation loading. In suchsystems, several options are available for achieving the necessarycooling, including integrating the cameras into dewars for liquidnitrogen or liquid helium, Stirling cryogenerators, Gifford-McMahonmechanical coolers, and other such devices.

To reduce thermal load on the cooling system, infrared camera designersoften place all of the cooled elements inside a vacuum vessel. Withinthe vacuum vessel, the radiation shield and the photocell are maintainedat a low, sometimes cryogenic, temperature, based on the photocellrequirements and the desired performance. The vacuum vessel, (ifpresent) often constitutes a camera housing, which also often contains,or provides, a mounting apparatus for the infrared focusing lens. Theterm “lens” as used herein should be understood to be inclusive of alllight collecting devices including refractive or reflective systems.

Thermal infrared cameras must be able to accommodate both hot and coldtarget objects and scenes, while distinguishing the target frombackground radiation. Although the thermal control methods describedabove can allow a camera to be used in a wide variety of thermal scenes,drastic changes in radiation quantities require different camerasettings. If the scene is too cool for ideal use with the camera, thecamera operator can take a longer exposure of the scene. Doing so mayadversely affect the frame rate and may lead to resolution problems ifthe camera or target is moving. Another solution typically used in theart is to change the electronic gain of the signal from the photocell,even though increasing the gain also increases the noise in theelectronic signal. Conversely, in hot scenes, reduced exposure time,reduced signal gain, or a combination of the two can allow an infraredcamera to capture the scene. When a very bright event occurs (e.g., anexplosion, a launch of a missile) in a scene with a very high dynamicrange the photocell could saturate. A method to avoid saturation of thephotocell is by reducing the size of the optical aperture. Inconventional video camera, the iris mechanism is often coupled to thephotocell readout electronics, controlling the iris in response to theradiation intensity.

Apertures and Cold Stops. A cold stop is simply a temperature-controlledaperture. In its most basic form, the cold stop is a fixed aperture,similar to the apertures found in some disposable visible light cameras.Variable diaphragms (hereinafter used interchangeably with the term“iris”) for light cameras, including continuously variable and swappablefixed apertures, have been described in patent art for many years (seee.g., U.S. Pat. No. 24,356 to Miller and Wirsching in 1859, U.S. Pat.No. 582,219 to Mosher in 1897). The variable diaphragm works by allowingmore (or less) of the radiation (visible light, in the case of visiblelight cameras) that reaches the focusing lenses to pass through to thephotocell or film. The focusing lens receives radiation and focuses itbased on the distance from the radiation source to the lens and theprescription of the lens. The prescription includes the focal length andthe f-number. In conventional visible light cameras (and unlike infraredcameras), the aperture is typically built into the compound lensassembly. That aperture then lets pass a certain desired portion of theradiation intercepted by the lens.

With a very large aperture, nearly all of the light arriving at thefocusing lens passes through the aperture. By reducing the size of theaperture, the mechanism of the aperture blocks a portion of the lightfrom entering. In typical visible light cameras, the aperture is locatedat the point where the cone of light from the object is wide, at thepupil or aperture stop; and thus diminishes the light intensity withoutaffecting the image quality. Lenses may have specific aperturerequirements, which determine the optimum position and size of theaperture. This is typically a function of the f-number (hereinafterinterchangeably also referred to as “f-number”), the focal length of thelens, and the construction.

However, in infrared cameras, the aperture cannot be located in thelens, since the lens is not cooled and the aperture must be cooled sothe aperture doesn't radiate onto the infrared photocell. A lens for usewith infrared cameras ideally has its exit pupil located far enoughbehind the lens mount to the camera body, and at the end of theradiation shield, where the cold stop is mounted. The fixed aperture istypically located in the converging path of the light at the end of theradiation shield; that is, between the lens and the focal plane. Theaperture thus defines an effective f-number for the system. If the lensf-number matches the fixed aperture f-number the camera is said to beaperture matched. If the lens f-number is smaller, i.e., faster thanthat of the fixed aperture then some of the incoming radiation isclipped by the aperture, and if the f-number of the lens is larger,i.e., slower, than that of the fixed aperture, the photocell can “see”the mechanical structure of the camera and it receives undesiredradiation from the camera structure.

Interchangeable lenses will have different f-number's. Unless theaperture is changed, the f-number of the camera won't match the f-numberof most of those lenses. A need thus exists for an adjustable aperturethat can match the f-number of interchangeable lenses. That aperturemust be placed at the lens' exit pupil location, that is, inside thevacuum enclosure and at the end of the radiation shield, and should beadjustable to match the lens' f-number or the exit pupil size.

As a result, when interchangeable lenses of a different f-number areused with an infrared camera, the system f-number may not match thef-number of the lens. No solution heretofore existed in the prior art tothis problem prior to this invention. A variable diaphragm or aperture,however, is able to correct the foregoing situation and match the systemf-number to the specific lens in use.

U.S. Pat. No. 6,133,569 (the “'569 patent”) to Shoda and Ishizuyadiscloses a thermal infrared camera that appears to incorporate theabove-mentioned features. The '569 patent further notes the promisingidea of using variable diaphragms in thermal feedback infrared cameras,that is, in cameras with thermal sensors controlling cooling elements.Specifically, Shoda and Ishizuya suggest the use of an opticallyvariable diaphragm optionally thermally coupled to the infraredradiation shield, but without providing the reader with any tangibledetails beyond the basic thought. However, due to the limitationsdiscussed hereafter in regard to cooling the variable diaphragm, the'569 patent does not make possible the use of such a variable diaphragmin an infrared camera.

The use of continuously variable diaphragms or swappable fixed aperturesthat are used to match interchangeable lenses with different f-numbernumbers in thermal infrared cameras hasn't been viable because offundamental packaging and thermal control problems. As earlierdescribed, the aperture must be cooled. While an effectively cooledvariable diaphragm is difficult to design, the problem becomesconsiderably more difficult if the aperture must be kept at cryogenictemperatures and be located inside a vacuum chamber. Within a vacuumchamber, the aperture and the associated drive mechanisms cannot outgas.Depending on the depth of vacuum, this may require a completely dry irisand specially designed lubricants, electrical wiring, motors, and gears.Moreover, the drive mechanism cannot add heat load onto the coolingsystem, nor allow conductive heat load from the ambient vacuum enclosureto affect the cooling system. Equally important, the aperture mustdissipate energy from the radiation that it blocks. These and otherconsiderations for the aperture itself have made implementing a variablediaphragm impossible given the prior art.

Further, with continuously variable diaphragms or interchangeable fixedapertures, there must be some mechanism for changing the size of theaperture. Mechanical, electromagnetic, piezoelectric, or other suchcontrol means must be available to change the diaphragm size orinterchange the fixed apertures. The control means must be strong enoughto operate the variable diaphragm or interchangeable fixed aperture in atimely manner, and either be thermally isolated from the photocell oroperate at cryogenic temperatures. If the aperture is in a vacuum, thecontrol means must be small enough to fit within the vacuum chamber orprovide some means for transferring mechanical force through the wall ofthe vacuum chamber. Where such transfer of mechanical force occurs,complex seals must be used to ensure the integrity of the vacuum isun-compromised and that excessive heat is not conducted into theradiation shield.

Aperture control means located in a vacuum chamber require constraintsthat make their implementation significantly less feasible. First, thematerials used in conjunction with the control means cannot outgas, asvaporized materials not only destroy the vacuum that provides thethermal isolation for the cold components, but also condense on thephotocell. For that reason, bearings, linings, coatings, windinginsulation, and any cements or glues must be eliminated or replaced witha fluorinated polymer or polytetrafluoroethylene based insulation, suchas Teflon® brand insulation, or otherwise be coated or manufactured withspecial non-outgassing materials.

Moreover, the motor control means must also be able to cool itselfeffectively without the typical convection of heat into air. This meansthat all heat generated in the motor must be dissipated throughconduction to the motor mounting apparatus. The control means musttherefore be thermally isolated from the aperture it controls. The motormust incorporate heat-reducing technology, including bipolar drives, lowcurrent standby systems, and other such options. Furthermore, thediaphragm control means must not produce electromagnetic interference(EMI) that can distort the electronic signal produced by the photocell.Mechanical or other temperature control means must often also beassociated with the motor.

Finally, for control means located in a vacuum, there is an additionalpotential problem created by high voltage to exposed conductors in themotor apparatus. In extremely low-pressure vacuums, the remaining airmolecules subject to high voltage can ionize and current will flow as ifthe vacuum chamber were an electron tube, creating strong coronaeffects. These effects are particularly problematic near highlysensitive photocells, so careful insulation is needed on any exposedelectric contacts.

An additional packaging problem exists where a variable diaphragm systemmust fit within the same confines as an existing fixed aperture camera.In these retrofit cases, the entire aperture control means must fitwithin very small confines that were not designed to accommodate suchhardware.

Accordingly, a need exists in the art for a variable diaphragm thatovercomes or avoids the above problems and limitations, whichconstitutes a principal object of the invention.

A further object of the invention is to allow the use of interchangeableoptics, including interchangeable compound lenses, in a single infraredcamera, by providing a means to match the aperture number (ie. f-stop)of the camera to the aperture number of the lenses.

A still further object of the invention is to retrofit an infraredcamera that contains a fixed cold stop aperture for use with a lens thatis of a different f-stop number from that of the camera.

Our prior application for U.S. patent Ser. No. 10/250,016, filed May 28,2003, presently pending, the content of which is incorporated herein,addresses most of the same goals, and describes an invention in athermal infrared camera that includes a variable aperture. Among otherthings, the present application describes a variable aperture assemblyof improved structure not previously described.

SUMMARY OF THE INVENTION

In accordance with the invention, a variable aperture assembly of athermal infrared camera integrates a truss, a rigid open framework,called the actuator, that's capped by an aperture ring and bottomed by adriving ring, and a radiation shield that contains an aperture ring atthe upper end. The radiation shield is located inside the frameworkpositioned with the two aperture rings juxtaposed. A plurality of smallblades positioned between the upper aperture rings collectively definesan iris or aperture. Opposite ends of the blades are respectivelycoupled to respective ones of the two aperture rings. One couplingallows pivotal blade end movement in one aperture ring and the othercoupling allows radial blade end movement in the other aperture ring,when one aperture ring is rotated relative to the other, changing thesize of the formed central iris or aperture much like the occurs with anordinary camera diaphragm.

With the radiation shield held stationary, rotating the driving ringrotates the aperture ring that's connected to the framework relative tothe companion aperture ring on the radiation shield, thereby pivotingthe overlapping blades outwardly or inwardly about one blade, dependingon the direction of rotation and varying the size of the definedaperture. The f-number of the infrared camera can thereby be adjusted tomatch the f-number of the object lens. The framework material is a poorheat conductor and, due to the skeletal nature of framework members, theheat transfer path between the driving ring and the aperture ring of theframework is of low thermal conductance so as to substantially thermallyisolate one end of the actuator from the other.

For a camera built with a hermetically sealed dewar or vacuum enclosurewith a fixed aperture that is to be used with a reflective telescopethat is not f-number matched to the camera, the present inventionteaches how to add an external variable aperture mounted in front of thecamera between the camera body and the telescope. The external variableaperture is placed in a vacuum enclosure and is cooled in a similarfashion to the variable aperture described herein that is located insidethe camera vacuum enclosure. The external variable aperture is coupledwith a relay optical assembly that images the variable aperture onto thecamera's fixed aperture, thereby matching the f-number of the telescopeto that of the camera.

Accordingly, with the invention a single thermal infrared camera may beused under a wide variety of target-scene radiation conditions that maybe rapidly changing, with interchangeable or zoom camera lensesrequiring matching or different size cold stops (f-numbers), and underother such dynamic situations. The aperture rings, blades and radiationshield can be cooled to cryogenic temperatures while heat from thedriving ring cannot readily propagate to the aperture ring.

A single thermal infrared camera under a wide variety of target-sceneradiation conditions that may be rapidly changing, with interchangeablecamera optics requiring different size cold stops, and under other suchdynamic situations. The invention makes possible the upgrading andretrofitting of fixed aperture infrared cameras with variable diaphragmhardware. The f-number of the camera may now be adjusted to optimizeimaging and the adjustment bridges the boundary between the very coldelements and those that are much higher in temperature withoutpermitting significant heat transfer that would adversely affectimaging.

The scope of application of the inventive method and apparatus isbelieved to be broad, as a number of alternative thermal isolation anddiaphragm control means may suggest themselves to those skilled in theart as suitable for a wide variety of thermal infrared cameraapplications. These applications include military thermal signatureidentification (including aircraft, vehicle, missile identification),military and other field of view changes (switching camera use from widearea search to narrow field of view as a target is acquired and tracked,used in target tracking and fire control systems), police surveillance(detecting the presence of people, objects, etc.), general security andsurveillance applications (detecting and identifying intrusions), searchand rescue (finding people or vehicles), firefighting (finding victimsin smoke-filled rooms), and general zooming in or out with infraredcameras, to name a few.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1, labeled “prior art” shows a schematic section view of aPeltier-cooled thermal infrared camera found in U.S. Pat. No. 6,133,569,which is helpful in describing the present invention;

FIG. 2A illustrates the path taken by light from an external object intoand within an infrared camera, namely through a focusing lens and anaperture to the infrared sensitive photocell, and illustrates the effectof various sized apertures on images and unwanted thermal loads ashelpful background to the invention;

FIGS. 2B and 2C respectively illustrate the path taken by light rays andan extraneous light ray through a camera in which the f-number of thecold stop of the camera is the same as the f-number of the object lensfor the camera, and through a camera in which the f-number of the coldstop of the camera is less than the f-number of the object lens for thecamera;

FIG. 3 shows a schematic view of a liquid nitrogen dewar based infraredcamera, showing a typical dewar in section;

FIGS. 4 a and 4 b, respectively, show a pictorial and schematic view ofthe principal components of a liquid nitrogen dewar based infraredcamera of the type in FIG. 3 that are located around the photocell andtheir relative assembled positions;

FIGS. 5 a and 5 b present, respectively, a pictorial layout and aschematic view of an embodiment of a variable aperture apparatus inaccordance with the invention, corresponding to the views of FIGS. 4 aand 4 b, respectively, highlighting the additions in implementing avariable diaphragm, the components necessary for a gear drivenembodiment and the approximate order of assembly;

FIGS. 6 a, 6 b, 6 c, 6 d, 6 e, 6 f, and 6 g show views of severalembodiments of the inventive aperture drive mechanisms, highlightingimplementations of the aperture with an exemplary worm gear drivenswappable fixed aperture drive in FIG. 6 a, a gear cog driven swappablefixed aperture in FIG. 6 b, a simple two-aperture worm gear drivenswappable fixed aperture drive in FIG. 6 c, an exemplary piezoelectricdriven swappable fixed aperture in FIG. 6 d, an exemplary piezoelectricdriven variable diaphragm in FIG. 6 e, and two exemplary embodiments ofelectromagnetic aperture control means in FIGS. 6 f and 6 g;

FIG. 7 is a schematic view illustrating sensor locations for logiccontrol systems that control the variable diaphragm;

FIGS. 8, 9, and 10 illustrate a practical embodiment of a variableaperture device for an infrared camera in top perspective view, sideview and bottom view, respectively;

FIG. 11 is a section of and FIG. 12 is an exploded view of theembodiment of FIG. 8;

FIG. 13 shows the variable aperture device of FIG. 8 as assembled withthree piezoelectric driving motors used to produce motion that changesaperture size;

FIGS. 14, 15 and 16 respectively pictorially illustrate a variety ofposition sensors that may be used in the variable aperture combinationof FIG. 8 to permit remote monitoring and indication of aperture size,including a Hall-effect sensor, a reflective optical switch and aslotted optical switch; and

FIG. 17 illustrates a fixed aperture camera that is retrofitted toinclude an external variable aperture device.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is of sufficient complexity that the many parts,interrelationships, process steps, and sub-combinations thereof simplycannot be fully illustrated in a single patent-type drawing or table.For clarity and conciseness, several of the drawing figures showparticular elements in schematic form and omit other parts or steps thatare not essential in that drawing to the description of a particularfeature, aspect, or principle of the invention being disclosed. Despitesuch omissions, as those skilled in the art should recognize, thefollowing description nonetheless enables one skilled in the art to makeand use the invention without undue experimentation, and severalembodiments, adaptations, variations, alternatives, and uses of theinvention are described, including those presently believed to be thebest modes of carrying out the invention. It should be appreciated thatthe details presented in this specification are presented as examples ofthe invention, and are not intended to be limiting the scope thereof.

The prior art infrared camera is typified by the U.S. Pat. No. 6,133,569(the “569 patent”) Peltier cooled thermal infrared camera, an embodimentof which is shown in FIG. 1 to which reference is made. The infraredcamera of FIG. 1 comprises all the basic parts of an infrared camera ofthe type that contains a vacuum chamber. From the outside, the camerahousing 34 supports a focusing lens 36 as the first part of the imageoptics system and cooling fins 14 to disperse the heat extracted by thecooling Peltier elements 10 and 12. A vacuum chamber 30 is containedwithin housing 34. That chamber contains a radiation transmissive window32 to allow infrared radiation to pass to the inner part of the infraredcamera. The vacuum chamber 30 also includes a variable aperturediaphragm mechanism 60 contemplated by the invention, illustrated inblock form, that was heretofore impossible to implement in an infraredcamera, is included. That mechanism corresponds to proposed elements 70and 71 suggested in the '569 patent, but not possible to implementwithout more information. Cooling Peltier elements 10 and 12 are locatedwithin vacuum chamber 30 and the separate cooling elements for theinfrared sensitive photocell 110, namely cooler 10, and for theradiation shield 40, namely, cooler 12. The radiation shield 40comprises an aperture 42 that allows infrared radiation to pass toinfrared radiation sensitive photocell 110.

Referring to FIG. 2A, a schematic view of radiation collected by afocusing lens 36 is shown that then focuses that radiation at thephotocell 110, through an aperture, illustrating how the aperture 41 isable to block too much (or too little) radiation, and demonstrating thebalance between vignetting and cold stop effectiveness. Two sizes of apossible aperture are shown: a large aperture 43, and a smaller one, 41.The radiation originates at the source 120, whether the radiation islight reflecting off an object, heat emanating from the object, or acombination of the two, is unimportant. Radiation from each point of thesource travels in many directions. Radiation (formed in cones 122 and124, drawn with dashed and dotted lines, respectively) arrives anywhereon the focusing lens 36. That radiation 122 and 124 is focused onto thephotocell 110, arriving at a location on the photocell relative to theoriginating source location to create an image of the original source120 on the photocell 110.

In this case, radiation cone 122 emanating from the circular shapedupper end of source 120 arrives at the bottom end of the photocell 110,where an image of that upper end is formed. The same focusing applies toradiation (cone 124, drawn with dotted lines) emanating from a point atthe bottom of the source. This radiation 124 focuses to the optics sideof the photocell 110. In this way, the entire source 120 is imagedupside down onto the photocell 110. The invention, however, is notlimited to applications with optics that invert the image. If a smallaperture 41 is used, the aperture blocks a portion of the radiationbeing focused toward the photocell and that portion of radiation cannotpass.

The aperture size is also dictated by the need to reduce or eliminateradiation emanating from non-cooled portions of the camera, because suchundesirable radiation would otherwise arrive at the photocell and causeinterference. This aspect of aperture size is shown in FIG. 2A, whereradiation cone 126 emanates from the camera housing to the right offocusing lens 36 in the figure. The two lines defining cone of radiation126 encompasses the relevant extremes of undesirable radiation. Thatcone of radiation is blocked by the small aperture 41, but is allowed toreach the photocell by the large aperture 43. By reducing the size ofthe large aperture 43 slightly, additional extraneous radiation 126 canbe blocked, without affecting the source radiation cone 122. By reducingthe size of the aperture 43 and impinging on the source radiation cone122, more of the radiation cone 126 from the non-cooled housing can beblocked. In this manner, between the too small 41 and too large 43apertures there's an appropriately sized aperture that offers the bestcompromise between vignetting and cold stop efficiency.

The difference between a matched lens and infrared camera and one not somatched is further illustrated in FIGS. 2B and 2C, respectively. Asillustrated in FIG. 2B, when the f-number of the lens 36, say f-3, ismatched to the f-number of the cold stop 41, also f-3, of the infraredcamera as example, the rays of infrared light coming from the distantobject being imaged proceed through the cold stop to the infrared sensor110 of the camera. However, a stray or extraneous ray 43 proceedsthrough the cold stop, but is not incident on infrared sensor 110.Hence, the extraneous ray does not adversely affect the received imageat the sensor. However, referring to FIG. 2C, when the f-number of thelens 36, say f-6, does not match the f-number of the camera, illustratedas f-3, the extraneous ray of light 43 proceeds through the cold stop ofthe camera and is incident of the infrared sensor 110. That adverselyaffects the image of the distant object at the sensor.

Referring again to FIG. 2A, further complicating aperture design,specific coatings are required for the photocell-side of the aperture aswell as the inside surface of the radiation shield 40, since radiationfrom cone 126 reflecting off the radiation shield 40 should be damped orabsorbed as efficiently as possible to reduce stray light load atphotocell 110. If the photocell-side of the aperture reflects radiation,stray radiation in the radiation shield 40 may be deflected back ontothe photocell 110.

The problem with aperture sizing shown in FIG. 2A remains, however, forcameras that change the f-number of the optics. The proper size aperturefor a camera depends on the f-number of the optics. Once that f-numberchanges, the aperture may no longer be optimal for the camera. Forexample, in applications where a wide angle search mode is used toacquire a target, an aperture may be optimal for a wide field of view.Once a target is acquired, however, the optics may be switched to anarrow field of view, using a zoom or telephoto lens as example. Thef-number thereby changes, changing the required cold stop size, andleaving the fixed aperture mismatched to the new optics. Reaction to theforegoing change is next addressed in the description of the best modeof implementing the invention.

Continuing the description of the prior art, internally, the varioustypes of prior art thermal infrared cameras are similar. FIG. 3 shows aschematic planar slice or section of a typical liquid nitrogendewar-based infrared camera. In this figure, the cooling element is aliquid nitrogen dewar 20 (hereinafter also referred to as an “LN2dewar”), pictorially illustrated. This is a chamber containing liquidnitrogen to which various parts of the infrared camera may be mounted,especially those requiring cryogenic cooling. Typical, though notrequired, of LN2 dewars, the camera housing 34 is integrated with vacuumchamber 28. The vacuum chamber 28 extends around the LN2 dewar 20 andthe radiation shield 40, so that everything within the exterior housing34 is within the vacuum chamber and under a vacuum 28. The LN2 dewar isfilled with liquid nitrogen and the photocell 110 is mounted directly tothe LN2 dewar 20. In this manner, the photocell 110 is maintained atcryogenic temperatures. The radiation shield 40 is also mounted to theLN2 dewar 20, to keep the radiation shield 40 at a similar temperature.Attached to the outside of the camera housing 34 is the external opticshousing 38, containing the focusing optics 36. The vacuum in the vacuumchamber is maintained by the transmissive window 32.

Reference is next made to FIG. 4 a, which shows a pictorial view of anexemplary housing and radiation shield of a liquid nitrogen dewar basedinfrared camera of the type described in FIG. 3. The portion of thecamera housing 34 that covers the optics section (as opposed to the LN2dewar section) in FIG. 4 a, is shown from the side that normally facesthe camera interior. At the center of the housing 34 is the transmissivewindow 32. The inside wall of the camera housing 34 makes up the wall ofthe vacuum chamber 30. The radiation shield 40 is shown from an aboveangle. The fixed aperture 42 of the prior art is located at the opticalentrance and center of the radiation shield 40.

FIG. 4 b is a side view planar slice of the assembled components of acamera, showing relative locations of the components. The photocell 110is shown below the radiation shield 40. The radiation shield 40 mountson the same plane as the photocell 110, both mounting onto the LN2 dewar(not shown). The optical entrance of the radiation shield 40 is thefixed aperture 42. The camera housing 34, including the radiationtransmissive window 32, is located exterior to radiation shield 40. Thearea around the exterior of radiation shield 40 is under a vacuum 28.

In order to implement a variable diaphragm, several new mechanisms arenecessary. FIG. 5 a shows a schematic layout view of an embodiment ofthe apparatus, arranged as in FIG. 4 a, but demonstrating some of themajor differences involved in implementing a variable diaphragm orinterchangeable aperture. As those skilled in the art appreciate, theforegoing reference to variable diaphragm implies one that iscontinuously variable. In the generic sense, however, the termencompasses both diaphragms with an aperture that may be changed in sizecontinuously or an aperture that may be changed in large steps (e.g.,swappable or interchangeable apertures as variously termed). Forconvenience the term is used herein in the generic sense, therebyencompassing interchangeable apertures. In order to accommodatemechanical aperture control means in a camera, the vacuum chamber 30 andhousing 34 must be modified to add either a motor control means mountinglocation 84 or means for transferring mechanical force through the wallof the vacuum chamber (e.g., a vacuum feed-through), also at location84. Two exemplary worm gear systems 62 comprised of several shown partsare directly exterior to location 84.

Both worm gear examples use a worm gear screw 66 attached through acoupler 70 to the control means. In the case of the top system, thevacuum chamber seal 78 is penetrated by a rotary dial adjuster 76 and avacuum feed-through 80. In this example, the dial adjuster 76 contactswarm air outside of the housing 34. Since the coupler 70 and the rest ofthe worm gear system are within the vacuum chamber 30, the dial adjuster76 and coupler 70 must be made of minimally heat-conductive materials orthermal insulator material. Moreover, in either the manual or motorizedworm gear examples, the coupler 70 for the dial adjuster 76 mustminimize the heat transference to the variable diaphragm 46. Inaddition, there must be a high performance vacuum seal for the vacuumfeed-through 80, capable of maintaining high vacuum. One of the severalseals necessary is shown 82. This system allows the infrared cameraoperator to adjust the aperture size manually and directly.

The second worm gear system 62 shown utilizes a motor drive 72 ratherthan a rotary dial adjuster 76. The motor drive 72 is attached to thevacuum chamber seal 78 and the coupler 70. Through the coupler 70, themotor drive 72 turns the worm gear screw 66. In this arrangement, themotor drive 72 must be vacuum-capable, which means that the motor drivemust not outgas, must use special coatings, must be capable of sheddingheat through the vacuum chamber 30 and camera housing 34, and must notcontain exposed electrodes that may cause a corona effect.

A further complication may arise when using a stepper motor 72 of theappropriate size. Such a motor 72 is not likely to possess sufficienttorque to operate variable diaphragm 46. Should that be the case, thesystem contemplates the use of a reduction gearbox. Such a gearbox wouldbe located between the motor 72 and the worm gear screw 66, where thecoupler 70 is shown in the figure.

In infrared cameras that do not integrate the camera housing 34 andvacuum chamber 30, it can be easier to maintain a cryogenic temperatureat the important parts of the camera. In such a case, the motor 72 couldbe mounted external to the vacuum chamber 30, to reduce heattransference to the cryogenic parts. Mounted externally, the motor 72would not need to be vacuum-safe, and could be a normal motor, simplyassociated with a vacuum feed-though of the type shown at 80. Such anembodiment simply drives the vacuum feed-through 80 and rotary dialadjuster 76 using an externally mounted motor.

In either arrangement, the worm gear screw 66 then contacts the drivengear 64 of the variable aperture assembly. This connection should be asthermally isolative as possible, using less thermally conductivematerials or insulators for the parts of the mechanisms. The driven gear64 is attached to the variable diaphragm 46 and the assembly is mountedto the optical entrance of the radiation shield 40 at aperture mountinglocation 44 (which is the location intended for a fixed aperture in anon-variable aperture design).

The typical variable diaphragm 46 consists of at least three basicparts. There are iris fingers or blades 54 and two rings 56 and 58 thatform the major components of a variable aperture mechanism. The irisblades 54 form smaller or larger apertures on the central axis as theyare manipulated. Generally, in some designs the iris fingers or blades54 are flat and either curved or triangular and have two pivot points.The two pivot points are attached to an inner ring 56 and an outer ring58. In the closed position, where the variable diaphragm size is at itssmallest (nearly completely closed), the pivot points form a line whichapproaches the center of the aperture. As the two rings 56, 58 rotaterelative to each other in opposite direction, the pivot points moveapart, causing the iris blades to pivot away from the center of theaperture, making the aperture size greater.

Variable diaphragm mechanisms of the foregoing construction are known inthe art and are available commercially. However, this mechanism allowsthe driven gear 64 to be attached in such a way that the whole worm gearsystem 62 can open and close the aperture. Although the best mode ofimplementation will vary by the application, one possible arrangement isfor the driven gear 64 to be attached to the inner ring 56 of thevariable aperture device (thus, here, the inner ring 56 has featuresidentical to the driven gear 64, allowing the two to be attachedphysically). The outer ring 58 is then attached to the radiation shield40 at the aperture mounting location 44. As the worm gear screw 66 turnsthe driven gear 64, the inner ring 56 is turned relative to the outerring 58, which is fixed. The variable diaphragm 46 is thus controlled bythe worm gear system 62.

The variable diaphragm 46 itself must meet certain requirements. Anyportion of the variable diaphragm 46 facing the photocell 110 should becoated in a radiation absorbing material or color. Typically, thephotocell-side of the radiation shield 40 is coated in black, though thereflectivity in the infrared is more important than the visible-spectrum“color.” The iris blades 54 must be allowed to move along one anotherfreely, without relying on greases or other outgassing lubricantmaterials. For this reason, the blades should be coated with a materialfunctionally similar to Teflon® brand polytetrafluoroethylenes. Theresult is that the iris blades 54 must be low friction and lowreflectivity coated. As a last caveat, the reflective and black coatingsof the iris blades 54 must be specifically designed not to shedreflective material onto the photocell-side of the overlapping irisblades.

FIG. 5 b shows the mechanism of FIG. 5 a, assembled and in planar slicedview, as in FIG. 4 b of the prior art. This figure shows the relativelocations of each of the parts shown in FIG. 5 a, as assembled. Here,the photocell 110 and radiation shield 40 would be attached to an LN2dewar (not shown). The variable diaphragm and driven gear are shownattached to each other in a single combined unit 52. The combined gearand aperture 52 is shown above the radiation shield 40 where it would beattached. The worm gear screw 66 can be seen adjacent to the combinedgear and aperture 52, where it can engage the gear and manipulate theaperture. The coupler 70 attaches the worm gear screw 66 to the vacuumfeed-through 80 and the rotary dial adjuster 76. The vacuum feed-through80 is shown penetrating the camera housing 34 into the vacuum 28.

FIGS. 6 a, 6 b, 6 c, 6 d, 6 e, 6 f, and 6 g show schematic views ofseveral embodiments of the inventive aperture drive mechanisms, shownwithout other parts of the infrared camera. For the sake of simplicity,the apertures and gears are shown as single combined units, though it isto be understood that they can be separate or joined units. Furthermore,although there are seven examples described herewith, these examples arenot limiting, and serve to teach the inventive apparatus and method.Additional embodiments will become obvious to those skilled in the art.

FIG. 6 a shows a schematic view of an exemplary gear driveninterchangeable fixed aperture. In this Figure, the worm gear 66 of thetype found in FIGS. 5 a and 5 b drives an interchangeable aperture wheel48. The aperture wheel 48 in this exemplary view has three fixedapertures of different size. When the worm gear screw 66 drives theinterchangeable aperture wheel 48, the various size apertures aresequentially positioned into the optical path, enabling one to selectthe best size aperture from those available.

FIG. 6 b shows in schematic view a gear driven example of the inventiveaperture system, as in FIG. 6 a, with a gear cog 68 instead of a wormgear 66.

FIG. 6 c shows a schematic view of a third embodiment of the system thatuses a swappable partial aperture wheel 50 that inserts a small aperturein front of a larger fixed aperture. This system would provide a switchbetween a larger aperture, the default aperture, not illustrated in thefigure, and the smaller size aperture in partial wheel 50 under thoseconditions that so warranted the smaller size. For example, in atargeting system that uses a wide field of view when scanning fortargets and, once a target has been acquired, swaps optics to a zoomlens that inherently possesses a narrow field of view two aperture sizesmay be sufficient. Although shown with a worm gear screw 66, thestructure would also work with a gear cog of the type shown in FIG. 6 b.Further, the invention contemplates a swappable partial aperture wheelcomprised of a variety of shapes and configurations, not limited topartial circles. For example, where the “circle” is more roughlyrectangular in shape, with a pivot point either at an end or at themiddle, the aperture “wheel” can be referred to as an aperture stick. Inthe cases, such as FIG. 6 c, where an aperture is inserted into theoptical path, the aperture should be located closely to the fixedaperture beneath it, to minimize changes that would affect radiationshield efficiency.

FIG. 6 d shows a schematic view of a piezoelectric motor driveninterchangeable aperture. This embodiment is otherwise similar to theaperture of FIG. 6 a; however, this exemplary embodiment does not use agear on, or attached to, the aperture disc. Here, a piezoelectric motor90 contacts the outer ring 94 of the aperture wheel 48 with its piezodriving element 92. The outer ring 94 of the aperture wheel 48 isreplaced with a friction surface with a sufficient coefficient offriction (the current best mode for implementing this embodiment is touse a ceramic ring for the friction surface). That enables the drivingelement 92 to grip the periphery of the outer ring and push the ring ina rotational direction about the axis of the ring.

FIG. 6 e shows a schematic view of a piezoelectric motor driven variablediaphragm. As in FIG. 5 a, the variable diaphragm 46 can be attached tothe radiation shield through the inner ring 56, and the outer ring 58can likewise be turned by the piezoelectric motor 90 and piezo element92 to actuate the variable diaphragm 46. In the cases of piezoelectricmotors, the motors can be mounted as shown in FIGS. 6 d and 6 e, or themotors may be mounted beneath or above the aperture and may actuate thevariable diaphragm or swappable fixed aperture from the top or bottomsurfaces rather than the outside surface of the aperture.

FIGS. 6 f and 6 g show schematic views of magnetic control means.Magnetic control means offer several distinct advantages, one of whichis the absence of any necessity to physically intrude into the vacuumchamber (reducing the possibility of a vacuum leak) and possessing fewerparts likely to outgas after being placed into the vacuum chamber, whichdestroys the vacuum. The magnetic control means are thus most useful incases where highly sensitive photocells are used and temperature controlis of the utmost concern. FIG. 6 f shows a schematic view of magneticdrive system that uses a conventional motor system. The variablediaphragm 46 is as described before, and can be a swappable fixedaperture, as well, though not shown here. The outer ring 58 (or theouter edge of a swappable fixed aperture, not shown) has affixed to itat least one permanent magnet 96, with two shown here. The dashed linerepresents the vacuum chamber wall 30, outside of which is located themagnetic drive ring 98, with at least one permanent magnet 96 affixedthereto, here two. Any of the drive mechanisms described in thisinvention, or any other drive mechanism, can be used to drive themagnetic drive ring 98. In this figure, a worm gear 66 is used for thatfunction.

When the worm gear 66 turns magnetic drive ring 98, the magnetic fieldcreated by the permanent magnets 96 cause the permanent magnets 96affixed to the outer ring 58 of the variable diaphragm 46 to turn alongtherewith. That rotational movement of the outer ring 58 actuates thediaphragm to change the aperture size (in the case of a continuouslyvariable diaphragm) and/or changes the fixed apertures (as in the caseof a interchangeable aperture device) as described above.

FIG. 6 g, similarly, uses magnetic fields to turn the variable diaphragmor swap the fixed apertures. In this figure, the variable diaphragm 46has permanent magnets 96 affixed and is located within the dashed vacuumchamber wall 30. One or more electromagnets 100 are located exterior ofvacuum chamber 30, positioned directly outside f the non-magnetic vacuumchamber wall 30. As the current applied to the electromagnets 100increases, the magnetic field generated thereby changes and thepermanent magnets 96 are forced to move within the field and actuate thevariable diaphragm 46 or swap the fixed apertures (not shown).

Another possible configuration includes the use of a mechanical systemsuch as a belt or chain, either directly driving the variable aperturedevice or driving a pulley attached to the aperture device. In fact,many similar configurations may suggest themselves to those skilled inthe art for inclusion in the present invention.

In any of the interchangeable aperture designs, a detection means shouldbe included for determining when the interchangeable apertures are inposition above the underlying fixed aperture or hole in the aperturemounting location. The detection means can include detents that stop theaperture wheel as it rotates under the manipulation of the motor beingused to drive the wheel, or contacts on the disc that send a signal tothe motor control means as the contacts pass another electrical contactfixed to some non-moving portion of the thermal infrared camera.Furthermore, optical means can be used for positioning, as well as manyother possible methods of implementing such detection means, and suchmethods are also contemplated by the invention.

Reference is next made to FIG. 7, presenting a schematic of an infraredcamera system that incorporates several exemplary parts of a logiccontrol system to control the variable aperture. In order to avoidunnecessarily complicating the drawing figure, the aperture controlmeans and variable diaphragm are not illustrated in detail. The aperturecontrol means is simply shown as blocks 60. For completeness, the camerahousing 34 is also shown. The cone of radiation 122 entering the camerathrough the focusing lens 36 arrives at aperture 42 inside the housingand is partially obstructed by the aperture. A first sensor 112 islocated inside the housing between the focusing lens 36 and the aperture42, mounted near an edge of aperture 42. That sensor measures thequantity of radiation arriving at the illustrated location. The sensor112 is connected by a electrical lead 114 to the logic control module116 and sends information to the control module, as indicated by thearrowheads. The photocell 110 also provides an output that is alsocoupled (indirectly, but is shown for simplicity as directly connected)to the logic control module 116. A second additional sensor 112′ isshown adjacent to the photocell 110, also connected via electrical lead114 to the control module 116. Although both the sensors 112 and 112′and the photocell 110 are shown connected to the control module 116,those are each exemplary connections. Either one of these connections issufficient, as would be other similar connections. The inventioncontemplates one or more sensors being used in applications requiringthe optional logic control system 116. An output of that system iscoupled to aperture control means 60

With at least one sensor of 112 and photocell 110 connected, the logiccontrol system 116 can receive information on the quantity of radiationpresent and can apply a programmed algorithm to determine theappropriate size for the aperture 42. The logic control module 116 thenprovides a signal to cause the aperture control means 60 to change thesize of the aperture 42, as appropriate. If the sensor used is thephotocell 110 or any other sensor located within the radiation shield40, the logic control system 116 can be feedback based, so that as theaperture size changes, the data to the logic control module 116 changes.Generic input means 118 allows the user to directly modify the aperturesize manually.

The aperture size can also be changed via control logic that is tied tothe selection of the interchangeable lens. When the user switches fromsearch to track mode on the infrared camera and thus swaps another lenswith a narrow field of view and different f-number for lens 36, logiccontrol system 116 and variable aperture reacts accordingly and adjuststhe f-number of the aperture to properly match the f-number of thereplacement lens.

FIGS. 8 through 13 present a specific embodiment of a variable apertureassembly for an infrared camera constructed in accordance with theinvention, such as the infrared cameras of FIG. 6E, FIG. 1 and FIG. 7,earlier described. In the embodiment of FIG. 6E, a piezoelectric motor90 drives a ring 58 that's concentric with and attached to an inneraperture ring 56 of a variable aperture device of the associatedinfrared camera, not illustrated. Both those rings essentially lie inthe same plane. If due to space limitations, it is not possible toinstall the driving motor (or motors) and driving ring in the same planeas an aperture ring, then, if a viable camera is to result, the drivingring and motor (or motors) must be moved to a position spaced from theaperture ring. That problem is solved in the preferred embodimentpresented in FIGS. 8-13. FIGS. 8, 9, and 10 illustrate the variable(e.g. adjustable) aperture device in top perspective, side and bottomviews, respectively. FIG. 11 is a section of and FIG. 12 is an explodedview of the embodiment of FIG. 8. FIG. 13 shows the variable aperturedevice of FIG. 8 as assembled with three piezoelectric driving motors.

Reference is initially made to FIG. 8. A preferred structure of avariable aperture assembly is presented in this figure in a topperspective view. That assembly includes an open truss or framework 120and is sometimes referred to herein for reasons that are apparent as anactuator. As the skilled person is aware, a truss is an assemblage ofmembers (as beams) forming a rigid framework; and a framework is askeletal, open work or structural frame. The truss is formed of stiffthin metal members 121 that extend between and form a unitary integratedstructure with cylindrical ring 122 on the bottom end and a flatwasher-like shaped ring 123 on the upper end, a driven ceramic ring 125,a radiation shield 129, a second upper ring 127 that forms the upper endof the radiation shield, and a number of blades 128. For reasons thatbecome apparent, rings 123 and 127 may sometimes be referred to hereinas aperture rings. Upper ring 127 lies beneath blades 128 and, thus, isnot entirely visible in FIG. 8, but is visible in FIGS. 10, 11 and theexploded view of FIG. 12 to which one may make brief reference.Continuing with FIG. 8, in place in an infrared camera, the foregoingassembly is seated in the vacuum region or chamber of the camerahousing, behind the entry window, earlier described in connection withFIG. 7 with the radiation shield 129 positioned over the infraredradiation detecting photocell and with the centrally located passage oraperture in the assembly positioned coaxial with the entry window. Allof the foregoing rings are coaxial with one another.

As shown in the side view of FIG. 9 to which reference is made, truss120 in this embodiment is formed by a first group of members 121. Thosemembers define, form, triangles atop circular ring 122, are evenlydistributed end-to-end about the periphery of the circular ring (andabout radiation shield 129) and are slightly tilted inwardly so that theapex of all the formed triangles lies in a virtual circle. Anintermediate ring 130 lies on that circle and is attached to the apex ofthe underlying formed triangles. In this particular embodiment, thediameter of ring 130 is smaller than the diameter of ring 122, but thatrelationship can be varied. Another group of truss members, alsonumbered 121, forms a second group of triangles located on and attachedto the upper surface of ring 130. That group of formed triangles isdistributed evenly, end-to-end, thereabout the periphery of ring 130(and about radiation shield 129). The apex of those triangles lies inand defines a second virtual circle. The flat washer-like shaped ring123 on the top end of the truss framework lies on that second circle andis attached to the apex of the formed triangles. The foregoing truss isan integrated structure. Preferably, the complicated framework is formedfrom multiple parts that are welded or soldered together manually or isformed by conventional or electrical discharge machining.

Metal members or struts 121 are relatively stiff and are at least strongenough to support ring 123 and allow at least the entire truss assemblyand aperture ring to be rotated by turning the bottommost ring 122. Thestrut assembly is also designed to flex in the axial direction tocompensate for thermal expansion and/or contraction of the radiationshield while remaining torsionally stiff. In the assembled position inthe infrared camera, the bottom edge of the radiation shield 129 restson a cooled support member, not illustrated in the figure, and ismechanically fixed in position. That support member is maintained cold,which maintains the photocell, which the radiation shield surrounds,cold, and, indirectly, maintains certain components of the variableaperture device, namely elements 123, 127 and 128, cold. The metalmembers or struts of the truss are also relatively thin which increasesthe thermal resistance of the heat conduction path from bottom ring 122,which, as placed in the infrared camera housing, is not cooled, to theupper ring 123, which is cooled.

The material selected for metal members and the rings of the truss is apoor quality heat conductor or, better yet, a thermal insulator. Due tothe nature of the structure of the truss, the quantity of metal betweenthe bottom and top ends of the truss is designed to be minimal andthereby minimize heat transfer between those ends. A preferred metalpreferred for the foregoing is Titanium. Stainless steel is a lesspreferred alternative. Some composite materials could be used as analternative, but would require different manufacturing processes thanused for the metal ones. The truss structure serves as a transition orbridge between a warm region and a cold region in the infrared camerawithout adversely affecting the integrity of the cold region or imposingtoo great a thermal load on the refrigeration equipment responsible forcooling.

Additionally, because the preferred actuator truss employs a minimum ofmetal, the structure is lighter in weight than would be the case inwhich the actuator is constructed with metal walls that are a continuoussurface, as in less preferred embodiments of the invention. That addsweight and produces an actuator that is relatively heavy. Since thetruss structure hangs on the radiation shield and the infrared camera issubject to shock and vibration in use, a heavy mass for the actuatorwould be inappropriate.

As a preferred option, a narrow ceramic ring 125 that's slightly largerin diameter than ring 122 fits over and attaches to ring 122, along theouter periphery of the latter. The ceramic, e.g., aluminum oxide, is apoor thermal conductor and adds additional thermal resistance in thepath from a relatively warm (e.g., high in temperature) source of heatexternal to the ceramic ring at the bottom end of actuator 120, such asthe motor actuators that drive the rings in rotation, later described,to the refrigerated elements of the adjacent infrared camera structure.The outer periphery of ring 125 has a surface that is hard, does notsignificantly wear in use, and is low particulating. Ring 123, locatedat the top of actuator 120, overlies another flat ring that is locatedat the top of radiation shield 129, but is not visible in this figureand is visible in FIG. 12 to which the interested reader may brieflyrefer.

Reference is again made to FIG. 8 and to aperture ring 123. The ringcontains a number of short slots 132, only one of which is numbered. Theslots extend from the inside circular rim of the ring toward the outerperiphery of the disk, and are evenly spaced. One end of each slot opensinto the central opening in the aperture ring. In this embodiment theslots 132 are straight and are radially directed. In other embodimentsthe slots may be curved, referred to as tangential by those skilled invariable apertures. Half of those slots capture and function as a guidefor respective ones of the cylindrical pins 133 of the underlying blades128 that underlie the central passage in ring 123 and collectivelydefining the central aperture for the camera or lens. The blades arecaptured between ring 123 and the underlying ring 127 on the upper endof radiation shield 129. Those elements are better illustrated in theexploded view of FIG. 12 to which reference is again made.

The thin blades 128 underlie ring 123 and are sandwiched in partiallyoverlapping relationship between ring 127 and ring 123. Blades 128 formthe aperture for the infrared energy, much like the variable diaphragmaperture device or iris in a conventional 35 mm light camera. Fiveblades are shown, each of which is flat and narrow and curved in ashallow convex arc. Although space exists in the structure for tenblades, a preferred number, the complication of illustrating that numberof blades would likely detract from the description. Hence, only fiveblades are illustrated in the figure.

Each blade 128 contains a pair of small cylindrical pivot pins 131 and135, one located at each end. The first pivot pin 133 of the two isdirected in one direction, upward. The second pivot pin 135 of the pairis located at an opposite end of the blade and is directed in theopposite direction, downward. Aperture ring 127 carried on the radiationshield contains a series of ten cylindrical holes 137, each of which isdesigned to function as a socket to receive a respective one of thedownwardly directed pivot pins 135 of a blade. Holes 137 are slightlylarger in diameter than the associated pivot pin 135 to permit the pinto rotate in the hole with no friction or only minimal frictionalresistance.

The pivot pin 135 of each blade in the series is inserted in arespective hole 137 in consecutive order, whereby the blades partiallyoverlap. That assembly orients the upwardly directed pivot pins 131 ofthe blades to be evenly spaced on top of ring 127, spaced a distancesufficient to align with one of the slots 132 in aperture ring 123 ofactuator 120. Aperture ring 123 (and the actuator 120) is lowered overthe blades and the radiation shield 129, and the slots in ring 123 arecarefully slipped over the extending upper pins 131. As those skilled inthe art appreciate, the result of the rings and blade relationship is avariable aperture device. That is, by holding the radiation shield 129and hence, ring 127, fixed and rotating ring 123 in one direction aboutits axis one end of blades 128 is pivoted outwardly to increase thediameter of the central aperture formed by the blades. By rotating ring123 in the opposite direction instead, the blade ends are pivotedradially inwardly, closing down that formed aperture. As seen in the topview of FIG. 8, the overlapping blades 128 define a central aperturecoaxial with the principal axis of the aperture rings and the entireassembly. However, by shaping the blades properly the aperture can bemade square, rectangular and various other shapes to better match thebundle of the rays of radiation passing from the lens through theaperture to the photocell. When the ring 123 is turned to open theaperture, the pivot pins 133 of the blades move radially outwardly inthe associated slots, while the pivot pins 135 on the underside of theblade pivot in their respective socket 137.

For completeness, reference is made to the top perspective view of thevariable actuator assembly in FIG. 13. As earlier described in thisspecification, a variety of known means are available for adjustingaperture size, manually mechanically, magnetically or electrically,which in this embodiment amounts to rotating the aperture ring 122 (orthe attached ceramic ring 125) in one direction or the other, turningthe actuator 120 and ring 123 that moves aperture defining blades 128.One way was to use a piezoelectric motor to drive the lower ring (or theceramic ring, if used). Such kind of driver is ideally suited fordriving the variable aperture assembly of FIGS. 8-11. In FIG. 13, threepiezoelectric motors 140, 141 and 142 are evenly spaced about theperiphery of ceramic ring 125, placed in operative engagement with theceramic ring, and fastened in place to the support structure, notillustrated, of the infrared camera to fix the relationship.

Piezoelectric motors (also known as piezo motors) can be operated in avariety of configurations. For the implementations discussed here, thedescribed piezo motors are being operated as linear actuators. Linearpiezo actuators or motors operate by producing very small motions atvery high frequencies to achieve linear motion. Motion is achieved in adirection normal to the pre-load force of the motors. Piezoelectricmaterials produce a small change in length when subjected to a voltaicpotential. Several commercial implementations of piezo linear motors arecurrently available. In general, linear piezo motors produce motion bycoupling a high frequency varying normal force with oscillating motionperpendicular to the normal force at the same frequency. A phase shiftbetween the varying normal force and side motion produces aslip-grab-slip effect and generates force and the ensuing motion. Piezomotors can also be configured to produce rotary motion using similarmechanisms.

Linear piezo motors require a pre-load force normal to the desireddirection of motion for operation. The pre-load or normal force istypically greater than the linear tangential force generated by themotor. It is therefore advantageous to use multiple motors spacedsymmetrically in a ring such that the applied normal forces have a netsum of zero on the rotating stage. Doing so reduces the drag associatedwith radial loading of variable aperture mechanisms and the removesdesign complexities that would be required to support large radialforces.

Each such piezoelectric motor contains a pin 144 that projects from thefront end of the motor, a driving pin, visible in the figure only inmotor 141. That driving pin is spring-loaded and presses against ceramicring 125, placing the motors in operative engagement with the ring andfirmly holding the ceramic ring (and attached actuator) in angularposition, when the motors are deenergized.

Ceramic ring 125 in this figure includes various stops, such as stops145 and 146, which are optional and were not included in the otherfigures. Those stops may be included as an option to allow a pair ofpredetermined aperture positions to be selected. When the ring attains amaximum amount of travel, the stop abuts the motor, as example, whichblocks further rotation. An additional advantage of using the mechanicalstops is that the stress on the pins of blades 128 of the variableaperture is minimized.

Another preferred option, the surfaces of the variable aperture thatface the photocell, such as one side of ring 127 and the remaininginside surfaces of radiation shield 129 and blades 128 are coated in ablack color or other radiation absorbing material. The blades are alsolubricated with Teflon® brand polytetrafluoroethylenes, a lubricant, orequivalent lubricant, which does not outgas in vacuum.

Construction of the blades in the variable aperture device isself-evident to those skilled in the art, as example by stamping orpressing using a die. The rings used in the present embodiment of thedevice are formed integral with the respective actuator and radiationshield, forming an integrated assembly and the blades are installed andused in that construction. As one appreciates the foregoing assemblyeasily fits in the interior of an infrared camera, is an integratedstructure that is small in size and light in weight, and, with fewparts, should be reliable and relatively maintenance free. As examplethe embodiment of FIGS. 8-13 may be installed within an infrared cameraof the type illustrated in FIGS. 3 and 7. More importantly, the assemblyis small enough to fit inside hermetically sealed type of infraredcameras that use Stirling engine for cooling and are commonly used inmilitary applications, and in handheld commercial infrared camera thatdo not use LN2 for cooling.

In the foregoing embodiment of FIG. 8, the driving ring 122 is locatedin a position that is vertically spaced from aperture ring 123 (see FIG.9) and also from the circular ring 127 at the upper end of radiationshield 129 (see FIG. 11). As those skilled persons recognize in otherpractical embodiments, the truss 121 could be of a relatively planargeometry rather than the three dimensional shape of FIG. 9. In that casedriving ring 122 and aperture ring 123 would still be coaxial andeffectively thermally isolated from one another, but would be spacedapart by the difference in the diameter between the two rings. Althoughthe details of a camera structure permitting such a variation can'tpresently be visualized, the possibility nonetheless remains. As onealso appreciates, a great advantage of the foregoing embodiment is theability to permit some elements of the infrared camera (and the rings ofthe embodiment) to be cryogenically cooled, while other elements are not(or cannot be cooled), which is desired in the most sensitive infraredthermal imaging cameras. However, that is not a limitation on theapplication of the variable aperture device. The invention is functionaland sound even in less sensitive infrared cameras that do not requirerefrigeration. Be that as it may, though functional, as a practicalmatter, the device may be more costly than one would wish for the kindsof imaging accomplished with those less expensive and less sensitivecameras.

In the embodiment of FIG. 8, the actuator is a metal truss structure.The open spaces in the sides of the truss minimizes the weight of theactuator and the slender members provide a constricted thermal pathbetween the ends of the actuator that minimizes the conduction of heatbetween the lower and upper ends. The lightness of the actuator is dueprincipally because the structure has large openings between the metalmembers of the truss framework, and there are no solid or continuouswalls. As those skilled in the art realize, it is possible for otherembodiments to provide an actuator that contains solid or continuouswalls, forming the hour glass-like or horn-like geometry and use thatalternative actuator in the variable aperture device of the precedingfigures. That can be accomplished without obtaining the benefits of thedesign of FIG. 8, yet fall within the scope of the present invention. Asexample, an actuator with solid walls constructed of metal, such astitanium. As a practical matter that alternative is not desirable orpracticable.

The better alternative of is to provide an actuator with solid wallsthat secures the advantages of the truss. Specifically, such analternative configuration of the actuator may be formed of a compositematerial, such as fiberglass. The composite material is not a metal, buta non-metal, is light in weight and, typically, possesses low thermalconductivity and a low temperature coefficient of expansion, to minimizeany change of shape with temperature change while the formed shape issufficiently rigid to be self-supporting and carry an aperture ring,Collectively those properties appear to provide an equivalent positiveeffect. Although not shown in the drawing, one may readily visualize theactuator 120 of FIGS. 8-13 as having solid walls instead of the trussand avoid the necessity for an additional drawing.

Having demonstrated the feasibility and structure of an infrared camerawith an adjustable aperture, which can be automatically or manuallyadjusted by the user, then new problems arise. As example, with theforegoing the only mechanism for predetermining aperture position is viamechanical stops. Accordingly, as an additional feature applicant hasadded position monitoring of the aperture size and invented the loopcircuit structure that provides that information, which is presentednext. Position sensing of variable aperture positions may be achievedvia a multiplicity of contact and non-contact methods. Non-contactmethods are illustrated.

FIG. 14 shows the incorporation of a proximity sensor. This figureillustrates a hall effect sensor 145 which returns proximity andposition information due to a change in flux associated with a ferrousportion of a sensing ring 147 that is attached to the driving ring 125Aof the actuator. Driving ring 125A corresponds to the ceramic ring 125used in the embodiment of FIG. 8, which has been modified as describedin the preceding sentence. By design the degree of angular rotation ofring 125A from a base position is correlated to the size of the apertureformed and that is the algorithm installed in converter 149. The angleinformation output from hall sensor 145 is coupled to a converter 149which converts the sensor reading to aperture size information andplaces that information in the appropriate form of digital signals fordriving a digital LCD display, such as display 151. The converter 149and LCD display may be miniaturized and carried by the infrared camera.For ease of understanding the power supply and associated power circuitsfor the foregoing are omitted, but should be well known to those skilledin the art.

FIG. 15 is an alternative rotation sensor that may be used in place ofthe one illustrated in FIG. 14. This rotation sensor also modifies theceramic ring 125 that's attached to the driving ring 122 of actuator 120of FIG. 8 to provide a sensing ring 125B, containing alternatingabsorbing and reflecting stripes 153 located on the edge of the ring. Areflective optical switch 154 that consists of a photodiode 155(collecting light) and a light emitting diode 156 (LED) that suppliesthe light. These are often referred to as photoreflectors. By design thedegree of angular rotation of ring 125B from a base position iscorrelated to the size of the aperture formed and that is the algorithminstalled in a converter, not illustrated. The converter converts thesensor reading to aperture size information and places that informationin the appropriate form of digital signals for driving a digital LCDdisplay, not illustrated. As in the prior figure, the converter and LCDdisplay may be miniaturized and carried by the infrared camera.

An alternative method using the same hardware would simply sense the endor beginning of an absorbing or reflective stripe. Another method wouldbe to use multiple photoreflectors and several stripes, eachcorresponding to a discrete or set of discrete aperture settings.

FIG. 16 shows an angular position sensor that uses a slotted opticalswitch. This embodiment also modifies the ceramic driving ring toprovide a sensing ring 125C that contains a tab or set of tabs 157,representing angular position, connected to the sensing ring and anotherimplementation of a photodiode/LED combination 159. During rotation ofthe actuator and sensor ring 125C the tabs pass between the emitter andlight source thereby interrupting the light path. As in the priorembodiments, a correlation exists between a particular tab and theangular position. An appropriate converter and LCD digital display, notillustrated, obtain the output of combination 159 and convert that todigital information of aperture size. Slotted optical switches of theforegoing type are readily available as are photoreflectors and hallsensors.

Some lenses, like those found in reflective telescopes contain an exitpupil or aperture stop that's located in the front of the lens, and donot contain an exit pupil behind the lens inside a vacuum enclosure.Placing an aperture inside the vacuum enclosure in such a case does notserve as an effective aperture stop, and would either cause vignettingor pass undesired radiation to the photocell. As those skilled in theart appreciate from an understanding of the present invention, there area great many reflective telescopes that are in use today, whosereplacement would be cost prohibitive, and yet such telescopes wouldgreatly benefit from the inclusion of an adjustable aperture, such aspresented in this application. While the solution is not immediatelyapparent, the present inventors addressed and resolved those needs. Anadditional object of the present invention, thus, is to retro-fitexisting infrared cameras that contain a fixed cold stop aperture foruse with a lens that is not matched to the f-number of that cold stopand also to retro-fit an reflective telescope that does not contain anaperture stop in the space behind the infrared camera mounting.

A like situation exists for infrared cameras that are constructed with ahermetically sealed dewar or vacuum enclosure, containing a fixedaperture, that are to be used with a reflective telescope that is notf-number matched to the infrared camera. As one appreciates theforegoing optical system employed lenses of shaped glass or crystallinematerial. Some of those lenses may optionally be telescopic. Basically,those systems are referred to as refractive optical systems. But there'salso another class of telescopic lenses, more specifically mirroredoptics that collect and direct light rays from distant objects to aneyepiece or sensor. As example, the combination a large concavedish-shaped mirror containing a central aperture and a small convexshaped mirror located in front of the concave dish coaxial with theaperture in the latter. Light from distant objects are collected by thelarge mirror and focused on the small convex mirror. The light incidenton the small convex mirror is reflected through the aperture whereby theimage propagates to an infra red camera. Those reflective mirror systemsare referred to as reflective optical systems.

In accordance with an aspect of the invention the retro-fit device addsan external variable aperture in front of the camera, between the camerabody and the telescope. The external variable aperture is placed in avacuum enclosure and is cooled in a similar fashion to the aperture,described earlier herein, located inside the camera vacuum enclosure.This external variable aperture is then coupled with a relay opticalassembly that images the variable aperture onto the camera's fixedaperture. In that way the front telescope is thereby f-number matched tothe camera.

FIG. 17 shows an arrangement for matching an infrared camera 160 thatcontains a built in fixed aperture 161, located to the right side, to areflective or refractive lens system 162 shown in block form to theleft, that is of an f-number that is not matched to the camera f-number.That “mismatch” occurs if a new lens is provided for the camera toaccomplish some unrelated imaging problem, as example, using the camerain a situation that the original camera and lens were not intended to beused. For this retro-fit a lens assembly 163 is incorporated betweenlens 162 and camera 160.

That lens assembly that re-images the image that is formed by the frontlens via a variable aperture 170. The lens assembly contains a variableaperture 170, which is controlled in size by any of the means earlierdescribed. That external variable aperture is cryogenically cooled bymeans independent of the camera and exist in a dedicated vacuumenclosure 171 to minimize heat transfer by convection. A first lensgroup 164, located to the left of the variable aperture contains threelenses that relay the image from lens 162 to form a stop at the externalvariable aperture. A second lens group 165 to the right side of thevariable aperture also contains three lenses. The second lens groupreforms the image from the first lens group and recreates that image atthe object location 168 of two additional groups of lenses 166 and 167.

Effectively, lens groups 164 and 165 define a 1:1 image relay thatrecreates the image. Lens groups 166 and 167 form a demagnifying relay(in this case) that relays the image formed by the 1:1 relay onto thecamera photocell array, via the fixed aperture 161 in the camera. Thebest practical embodiment of the lens assembly for a specific infraredcamera and a specific reflective telescope is described in table 1 inthis specification.

In situations where an infrared camera is already built with a fixedaperture, and a need arises to use that camera with a reflectivetelescope (that does not have an exit pupil, or aperture stop at theposition of the fixed aperture, or with a lens that is not f-numbermatched with the camera an approach exist for building an externalvariable cold stop to the camera. The external variable cold stop andthe actuation mechanism can be any of those that were earlier hereindescribed. The variable aperture is placed inside a vacuum enclosure forreason discussed elsewhere in this specification.

This external-to-the-camera variable aperture cold stop serves to matchthe f-number of the telescope or lens, while relaying the images througha set of lenses properly designed to achieve several objectives: (1) theimage of the telescope or lens is relayed through intermediate imageplanes to the photocell on the camera, (2) the external aperture stop isreimaged onto the camera's fixed aperture stop. In doing so, in effectthe camera is being matched to the f-number of the lens.

The location of the external aperture behind the telescope is determinedby the optics between the telescope and the cold stop that form an imageof the telescope's aperture stop at the position of the variableaperture. This system solves the problem of using a single camera thathas a built in fixed aperture with many telescopes or different lensesthat are f-number unmatched to the camera.

The infrared camera diaphragm apparatus should have wide industrialapplicability, not only to military, police, search and rescueapplications, but also to other applications in which infrared signalsmay be present in a wide variety of quantities, such as in agriculture.The invention has further applicability in any circumstance where thedynamic range of the given electronics is insufficient, where variousf-numbers are needed, or where additional user tunability is desired.

It is believed that the foregoing description of the preferredembodiments of the invention are sufficient in detail to enable oneskilled in the art to make and use the invention without undueexperimentation. However, it is expressly understood that the details ofthe embodiment presented for the foregoing purpose is not intended tolimit the scope of the invention in any way, in as much as equivalentsto those elements and other modifications thereof, all of which comewithin the scope of the invention, will become apparent to those skilledin the art upon reading this specification. Thus, the invention is to bebroadly construed within the full scope of the appended claims.

TABLE 1 System optical details (example of one embodiment for matchingan infrared camera with an f number of f/4 to a two-mirror telescope off/10) Effective Focal Length: −33.77514 F/3.99 Thickness Diameter SurfRadius (mm) (mm) Glass (mm) conic Con Comments OBJ Infinity 0 25.5 0Object plane  1 Infinity 44.0 25.5 0  2 −2001.1 6.5 ZNSE 42 0 Lens 1  3−106.0 13.0 42 0  4 32.1 7.2 ZNSE 38 0 Lens 1  5 40.6 1.5 29.0 0  6 19.97.6 CAF2 30 0 Lens 3  7 12.7 16.5 19.6 0 STO Infinity 17.1 8.3 0External Variable Cold Stop  9 −12.7 7.6 CAF2 19.6 0 Lens 4 10 −19.9 0.830 0 11 −39.2 6.5 ZNSE 38 0 Lens 5 12 −31.0 12.5 30 0 13 111.3 6.5 ZNSE42 0 Lens 6 14 Infinity 45.7 42 0 15 Infinity 12.1 25.5 0 IntermediaryImage plane 16 36.5 4.8 ZNSE 34 0 Lens 7 17 27.9 54.5 29.4 0 18 Infinity10.1 SILICON 64 0 Lens 8 19 −123.5 1.1 64 0 20 42.8 10.5 SILICON 54.3 0Lens 9 21 72.0 1.81 46.7 0 22 103.4 4.9 GERMANIUM 50 0 Lens 10 23 37.923.5 37.2 0 24 162.4 5.8 SILICON 28 0 Lens 11 25 −396.2 12.4 28 0 26Infinity 1.0 GERMANIUM 10.0 0 Window 27 Infinity 0.4 9.8 0 28 Infinity0.5 SILICON 9.3 0 Filter 29 Infinity 0 9.2 0 30 Infinity 26.9 9.2 0Internal Fixed Cold Stop IMA Infinity 20.4 0 Image

1. A variable aperture assembly for an infrared camera, said infraredcamera for use with optics comprising at least two f-numbers, aradiation shield, cooling means, a central axis, a focal plane arraymounted substantially coaxial and substantially perpendicular to saidcentral axis, said radiation shield and said focal plane array beingthermally connected to said cooling means, said radiation shield havingan optical entrance end accepting infrared radiation passing throughsaid optics and a thermal connection end proximal to said focal planearray providing said thermal connection to said cooling means, saidradiation shield being positioned to block stray radiation from aninside surface of said infrared camera, and an optical pathsubstantially along said central axis, infrared radiation being admittedthrough said optics along said optical path, passing through saidoptical entrance end of said radiation shield and arriving at said focalplane array, said variable aperture assembly comprising: a variable irisdisposed substantially along said central axis; said variable irishaving an infrared radiation admitting central opening coaxial to saidcentral axis; said variable iris being located substantially at a sameposition along said central axis as an exit pupil of said optics,forming an aperture stop; said variable iris thermally connected to saidcooling means; said variable iris capable of causing said infraredradiation admitting central opening to vary in size; mounting means formounting said variable iris; said mounting means allowing said thermalconnection to said cooling means; said mounting means positioning saidvariable iris proximal to said optical entrance end of said radiationshield; said mounting means being designed to minimize deflection ofsaid variable iris, causing said variable iris to remain substantiallyparallel to a plane of said exit pupil; actuation means for controllingsaid size of said infrared radiation admitting central opening of saidvariable iris; thermal isolation means for connecting said variable iristo said actuation means; said thermal isolation means comprising asufficiently rigid structure designed to reduce conductive heat transferfrom said actuation means to said variable iris; said sufficiently rigidstructure being sufficiently strong to minimize flexion of said thermalisolation means caused by said actuation means; wherein said variableaperture assembly provides f-number matching by adjusting said infraredradiation admitting central opening at least small enough to effectivelymatch a highest f-number of said optics and large enough to effectivelymatch a lowest f-number of said optics.
 2. The variable apertureassembly of claim 1, wherein said variable iris is continuouslyvariable.
 3. The variable aperture assembly of claim 1, wherein saidvariable iris comprises at least one position where said infraredradiation admitting central opening comprises a non-circular shape. 4.The variable aperture assembly of claim 3, wherein said non-circularshape comprises a generally rectangular shape.
 5. The variable apertureassembly of claim 4, wherein said generally rectangular shape comprisesa rectangle with rounded edges.
 6. The variable aperture assembly ofclaim 4, wherein said generally rectangular shape comprises a racetrackshape.
 7. The variable aperture assembly of claim 1, wherein saidvariable iris comprises a plurality of aperture blades, said pluralityof aperture blades arranged to define said size and a shape of saidinfrared radiation admitting central opening.
 8. The variable apertureassembly of claim 7, wherein said plurality of aperture blades comprisesfour aperture blades, and wherein said variable iris comprises at leastone position where said infrared radiation admitting central openingcomprises a generally rectangular shape.
 9. The variable apertureassembly of claim 1, wherein said variable iris capable of causing saidinfrared radiation admitting central opening to vary in size, forms saidinfrared radiation admitting central opening in at least two discretesizes.
 10. The variable aperture assembly of claim 9, wherein said atleast two discrete sizes provide at least two aperture stops in sizesthat match said at least two f-numbers of said optics.
 11. The variableaperture assembly of claim 1, wherein said radiation shield of saidinfrared camera comprises a first fixed opening at said optical entranceend, said first fixed opening disposed to form an aperture stop of asize to match a numerically lower of said at least two f-numbers, saidvariable aperture assembly comprising: said variable iris capable offorming said infrared radiation admitting central opening into a secondopening, said second opening being smaller than said first fixedopening, to match a numerically higher of said at least two f-numbers;said actuation means causing said infrared radiation admitting centralopening to vary in size from said smaller size of said second opening toa size larger than said first fixed opening in said radiation shield;wherein said variable iris provides said aperture matching for saidoptics for any of said at least two f-numbers except for saidnumerically higher of said at least two f-numbers, and wherein saidradiation shield provides said numerically lower f-number matching. 12.The variable aperture assembly of claim 11, wherein said infraredradiation admitting central opening of said variable iris and said firstfixed opening comprise generally non-circular shapes.
 13. The variableaperture assembly of claim 12, wherein said generally non-circularshapes comprise generally rectangular shapes.
 14. The variable apertureassembly of claim 13, wherein said generally rectangular shapes compriserectangles with rounded edges.
 15. The variable aperture assembly ofclaim 13, wherein said generally rectangular shapes comprise racetracks.16. The variable aperture assembly of claim 12, wherein said variableiris comprises of a plurality of aperture blades, said plurality ofaperture blades arranged to define said size and said shape of saidinfrared radiation admitting central opening.
 17. The variable apertureassembly of claim 14, wherein said plurality of aperture bladescomprises four aperture blades, and wherein said variable iris comprisesat least one position where said infrared radiation admitting centralopening is generally rectangular in shape.
 18. The variable apertureassembly of claim 1, wherein said variable iris prevents entrance ofsaid radiation through said infrared radiation admitting central openingby closing completely said infrared radiation admitting central opening.19. A multiple discrete f-number variable aperture assembly for aninfrared camera for use with optics comprising multiple discretef-numbers, said infrared camera comprising a radiation shield, coolingmechanism, a central axis, a focal plane array mounted substantiallycoaxial and substantially perpendicular to said central axis, saidradiation shield and said focal plane array being thermally connected tosaid cooling mechanism, said radiation shield having an optical entranceend accepting infrared radiation passing through said optics and athermal connection end proximal to said focal plane array providing saidthermal connection to said cooling mechanism, said radiation shieldbeing positioned to block stray radiation from an inside surface of saidinfrared camera, and an optical path substantially along said centralaxis, infrared radiation being admitted through said optics along saidoptical path, passing through said optical entrance end of saidradiation shield and arriving at said focal plane array, said multiplediscrete f-number variable aperture assembly comprising: at least twodiscrete apertures disposed substantially along said central axis; saidat least two discrete apertures having infrared radiation admittingcentral openings coaxial to said central axis; said at least twodiscrete apertures being located substantially along said central axisproximal to a plane of an exit pupil of said optics, forming an aperturestop; said at least two discrete apertures thermally connected to saidcooling mechanism; said at least two discrete apertures capable ofcausing said infrared radiation admitting central opening to vary insize between sizes of said at least two discrete apertures; mountingapparatus for mounting said at least two discrete apertures; saidmounting apparatus allowing said thermal connection to said coolingmechanism; said mounting apparatus positioning said at least twodiscrete apertures proximal to said optical entrance end of saidradiation shield; said mounting apparatus being designed to minimizedeflection of said at least two discrete apertures, causing said atleast two discrete apertures to remain perpendicular to said centralaxis; a selection mechanism configured to cause one of said at least twodiscrete apertures to provide said infrared radiation admitting centralopening; said selection mechanism configured for thermal isolation; saidselection mechanism comprising a substantially rigid structure designedfor high thermal isolation, to reduce conductive heat transfer from saidselection mechanism to said at least two discrete apertures; saidsubstantially rigid structure to minimize flexion; wherein said variableaperture assembly provides f-number matching by selecting said infraredradiation admitting central opening from said at least two discreteapertures, selecting one of said at least two discrete apertures toeffectively match a highest f-number of said optics or one of said atleast two discrete apertures to effectively match a lowest f-number ofsaid optics.
 20. The multiple discrete f-number variable apertureassembly of claim 19, wherein one of said at least two discreteapertures formed in said radiation shield cause said optical entranceend of said radiation shield to be a fixed aperture to provide aperturematching to said lowest f-number of said optics.
 21. The variableaperture assembly of claim 20, wherein said infrared radiation admittingcentral openings comprises a non-circular shape.
 22. The variableaperture assembly of claim 21, wherein said non-circular shape comprisesa generally rectangular shape.
 23. The variable aperture assembly ofclaim 21, wherein said central openings are generally racetrack inshape.
 24. The variable aperture assembly of claim 19, wherein at leastone of said infrared radiation admitting central openings of said atleast two discrete apertures comprises a non-circular shape.
 25. Thevariable aperture assembly of claim 24, wherein said non-circular shapecomprises a generally rectangular shape.
 26. The variable apertureassembly of claim 25, wherein said generally rectangular shape comprisesa rectangle with rounded edges.
 27. The variable aperture assembly ofclaim 25, wherein said generally rectangular shape comprises a racetrackshape.
 28. The variable aperture assembly of claim 19, wherein saidvariable iris prevents entrance of said radiation through said infraredradiation admitting central opening by closing completely said infraredradiation admitting central opening.